S1861
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
are limited to tube potentials of ~50 kV, the Vektra operates up to 100 kV. In the Vektra magnets steer electrons off-centre onto a tungsten target with molybdenum septa shown in Figure 1 [1]. This generates an anisotropic beam that conforms dose to the tumour, while sparing nearby healthy tissues. Characterization and dose-to-water calculations for a 27 mm diameter Vektra tube at 50, 85, and 100 kV have been reported[2], but a second Vektra design with a 17 mm diameter tube and up to 100 kV has not yet been similarly evaluated. This study models the 17 mm source using vendor-supplied computer-aided design (CAD) files to realistically simulate the x-ray spectrum at50, 60, 70, 80, 90, and 100 kV, with future comparisons planned against spectrometry measurements to support commissioning and quality assurance for clinical implementation.
Material/Methods: The CAD files and material composition data were used to model the Vektra source[3], including its 6 μ m tungsten target, 17 mm tube diameter and internal geometry, within a Geant4-based usercode named VektraMC. An incident electron beam was simulated, and interactions within the target produced bremsstrahlung and characteristic x-rays. To improve efficiency, bremsstrahlung splitting was implemented in the target region. The energy, position, and momentum of emerging photons were scored at the device tip and stored as phase-space files. Spectra were generated for all available beam energies (50, 60, 70, 80, 90, and 100 kV). Next, these phase-space distributions will be used to calculate dose to water in a voxelized phantom. Spectrometry and ion-chamber measurements will be performed to compare the measured and simulated spectra and dose. Results: Figure 2 shows the normalized x-ray spectra generated for electron beams of 50 to 100 kV, with bin widths of 0.1 keV, and their mean energies. The characteristic x-rays of molybdenum (K α 1=17 keV and K α 2=19 keV) are observed for all beams. The x-rays for tungsten (K α 1=59 keV and K α 2=67 keV) are observed for electron beams with energies above 60 kV.
Conclusion: By excluding a 3 mm rim of connective tissue around the GTV (-Conn.Tissue), it was possible to improve the target coverage for the majority of patients without compromising the dose to the OARs. Keywords: MR-linac, Adaption, Ultra-Central lung tumors Digital Poster 1907 Spectral and Dosimetric Validation of the Vektra(TM) Electronic Brachytherapy System with Monte Carlo Simulations 1 Department of Oncology, Medical Physics Unit, McGill University, Montreal, Canada. 2 Lady Davis Institute for Medical Research, Jewish General Hospital, Montreal, Canada. 3 Empyrean Medical Systems, Inc, Boca Raton, USA Purpose/Objective: The Vektra(TM) Electronic Brachytherapy system (Empyrean, Boca Raton, FL) delivers directional x-ray beams with higher-energy spectra than previous electronic brachytherapy sources. While other sources Emma Raleigh-Smith 1,2 , Dirk Bartkoski 3 , Azin Esmaelbeigi 1,2 , Jonathan Kalinowski 1,2 , Shirin Abbasinejad Enger 1,2
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